1. Field of the Invention
The present invention relates generally to a tracking error generating device and, more particularly, to a tracking error generating device for use in a reproducing apparatus for reproducing an optical disc.
2. Description of the Related Art
As tracking methods suitabe for optical disc players, a variety of methods have been employed, including a differential phase detection (DPD) method, a 3-beams method, a push-pull method, and so on. Since the differential phase detection method is capable of providing a tracking error signal even at higher track density as compared with the 3-beams method and the push-pull method, it has recently been employed in the DVD (Digital Versatile Disc or Digital Video Disc) players.
When recording digital information data on the optical disc, the information data is generally modulated using an RLL (Run Length Limited) coding and the coded information data is recorded on the optical disc in the form of marks (or spaces) of different length. In the DVD recording, for example, recording marks having a variety of lengths ranging from 3T (where T indicates a channel clock period) to 11T using an 8/16 modulation. It is however expected that the minimum pit length and hence amplitude of reproduced signal form the pits decreases with the evolution of higher density recording of the optical disc. Therefore, a problem will arise that the tracking performance in the phase differential detection method deteriorates owing to the decrease of the amplitude of reproduced signal.
FIG. 1 schematically illustrates an exemplary configuration of a tracking error generating device according to a conventional phase difference detection method. Referring specifically to FIG. 1, a quadruple detector 11 is composed of four light receiving elements (i.e., A, B, C and D) that are divided by a boundary parallel to a tangential direction of a recording track and a boundary orthogonal to the tangential direction. Within the quadruple detector 11, output signals Ra, Rd from a pair of the diagonally positioned light receiving elements A, D are summed by an adder 21, while output signals Rb, Rc of a pair of diagonally positioned light receiving elements B, Care summed by an adder 22. The resultant sum signals are supplied to a phase difference detector 23. The phase difference detector 23 detects a phase difference .DELTA.t between these two sum signals (Rat+Rd) and (Rb+Rc) . The phase difference signal output from the phase difference detector 23 is smoothed in a low pass filter (LPF) 25 which consequently generates at its output terminal a tracking error signal having a voltage proportional to the phase difference.
Another example of a method for obtaining a phase difference as described above is as follows. The quadruple detector detects a phase difference between respective output signals of light receiving elements A and B opposing each other with respect to a boundary parallel to the tangential direction of a recording track, and a phase difference between respective output signals of light receiving elements C and D in a similar positional relationship, respectively. Then, the phase difference calculating circuit calculates the difference between the two phase differences thus detected. The calculated difference is used as a phase difference signal.
A further example of such a method for obtaining a phase difference is as follows. The quadruple detector detects four signals each indicative of a phase difference between a sum signal (Ra+Rb+Rc+Rd), which is the sum of respective output signals of four light receiving elements, and each of the output signals Ra, Rb, Rc, Rd of the four light receiving elements. Then, the phase difference calculating circuit sums the phase differences of outputs in each of two sets of light receiving elements disposed at diagonal positions, and subtracts one addition result from the other. The calculated difference is used as a phase difference signal.
Conventional tracking error generating devices as mentioned above have a problem in correctly detecting the phase difference for short recording pits as a line recording density of an optical disc is increased, thus failing to generate a correct tracking error signal.
FIGS. 2A and 2B are waveform charts for schematically showing output signals a1, a2 from light receiving elements which are input to a phase difference detector 23, and a phase difference between these output signals a1, a2. For example, a1 corresponds to (Ra+Rd); and a2 to (Rb+Rc) in FIG. 1.
FIG. 2A illustrates output signals of light receiving elements when long marks (or spaces) are being tracked. In this event, the output signals of the light receiving elements have suffiently large amplitudes and wide time widths between zero cross points, so that the phase difference can be correctly detected as a time difference between zero cross points of the respective signals. On the others hands, when short marks (or spaces) are being tracked as illustrated in FIG. 2B, the output signals of the light receiving elements have small amplitudes and narrow time widths between zero cross points, so that zero cross points cannot be reliably detected. Furthermore, in some cases, the output signals may not cross the zero level. Thus, the conventional tracking error generating device has a problem in that it cannot correctly detect the phase difference in some cases, and therefore fails to generate a correct and accurate tracking error signal.